Chemically sensitive field-effect transistors (CHEMFETs) have been developed for the detection of specific compounds in liquid and gaseous environments. Originally, these CHEMFETs were ion sensitive, as disclosed in U.S. Pat. No. 4,020,830 to Johnson et al. entitled "Selective Chemical Sensitive FET Transducers" and by Piet Bergveld, "Development, Operation, and Application of the Ion-Sensitive-Field-Effect Transistor as a Tool for Electrophysiology", IEEE Transactions of Biomedical Engineering, pages 342-351, September, 1972.
Other developments produced CHEMFETs specifically capable of measuring the concentrations of components in a gaseous state, as for example the device disclosed in U.S. Pat. No. 3,719,564 to Lilly, Jr. et al.; the device described by Lundstrom in "A Hydrogen-Sensitive MOS Field-Effecting Transistor", 26 Applied Physics Letters, pages 55-57, Jan. 15, 1975; and the device described by G. Phillips entitled "An Electronic Method of Detecting Impurities In The Air", Volume 28, Journal of Scientific Instrumentation, pages 342-347, 1951.
A further advancement occurred with the disclosure of suspended gate field-effect transistors by Jiri Janata in U.S. Pat. Nos. 4,411,741 and 4,514,263, both of which patents are incorporated herein by reference. These patents describe an apparatus and method for measuring the concentration of various components in a liquid or gaseous fluid sample. The apparatus comprises a chemically sensitive field-effect transistor having a semiconductor substrate and a pair of diffusion regions formed at the surface of the substrate. An electrical insulating layer is positioned adjacent the substrate and a fluid pervious gate member is mounted to the insulating layer so as to form a gap between the suspended gate and insulating layer. The apparatus also includes means for imposing an electrical charge on the suspended gate, means for imposing an electrical potential between the diffusion regions, and means for detecting current flow between the diffusion regions. The fluid sample to be analyzed is introduced through the suspended gate and into the gap where various components of the fluid sample are adsorbed by the suspended gate, and in another embodiment, also by an adsorptive layer which is applied within the gap. The suspended gate and the adsorbtive layer can be specifically chosen so as to render the apparatus chemically selective to one or more specific fluid components.
The suspended gate field-effect transistor described by Janata above may provide a route to novel gas detection devices such as the miniature and portable gas detection device described in U.S. Ser. No. 811,548, filed Dec. 20, 1985, entitled "Method and Device for Detection of Changes in Gas Concentration". However, it has been found that the characteristics of suspended gate field-effect transistors having noble metal suspended gates are quite irregular and vary widely from SGFET to SGFET. The same suspended gate field-effect transistor may not be capable of reproducible sensing, and the responses from SGFET to SGFET may vary in an unpredictable manner. Such variation makes suspended gate field-effect transistors unsuitable for use as gas detection devices. What is needed is a method of forming suspended gate field-effect transistors having stable and uniform gas detection characteristics.
Thus, it is an object of the present invention to provide a method for forming suspended gate field-effect transistors having desirable gas detection characteristics and substantially uniform sensitivities from SGFET to SGFET.
It is another object of the present invention to provide a method of regenerating a chemically sensitive field-effect transistor having a suspended gate.
These and other objects of the present invention will become apparent to one skilled in the art from the below description of the invention and the appended claims.